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[equinox-dev] Fw: [platform-swt-dev] Sandboxed SWT. Is this possible?


An interesting thread on the SWT mailing list...
Jeff

"Ivan Markov" <ivan.markov@xxxxxxxxxx>
Sent by: platform-swt-dev-bounces@xxxxxxxxxxx

09/19/2005 05:03 AM

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[platform-swt-dev] Sandboxed SWT. Is this possible?





Hello everybody,
 
Inspired by some constructive criticism on a well known Java forum, I'm recently thinking of how much effort it would be to achieve code based security in SWT. I will use my SWT/Fox port as a playground.
 
Your thoughts on the following questions are very welcome:
 
Would it be useful? Perhaps, yes. SWT is probably the single Java API that currently cannot be sandboxed due to the exposure of various "crash-friendly" things like OS.java, Control.handle, etcetera.
 
Would this be easy to achieve, without sacrificing the simplicity of the code and without the need for total rework of the SWT internals? Perhaps, yes. Please, read on.
 
My plan is as follows:
There are several main sources of crashes in SWT:
 
1. The native layer (OS.java and friends) is exposed for everyone to use. This is something I'm able to tolerate as otherwise probably we need tons of abstractions or something related to C++'s "friend" construct in order to cross package-level boundaries and still preserve private visibility. Note that there is already a JSR which is supposed to bring a similar facility to Java. The main problem with the exposed native layer is that everyone is able to call a static function in OS.java with bogus parameters provided, thus crashing the JVM process. If this is remedied somehow, the exposure of "support" structures like LOGFONT, LOGPEN, BITMAP etc. should not be a problem: the important thing to grok here is that user may create and change these as he wills, but as long as he is not allowed to directly call functions of OS.java, he can't crash the JVM, as OS.java represents the "code" of the operating system, whiule everything else is the "data".
 
One way OS.java can be protected is by a) marking all natives as package protected or private and b) creating non-static public wrapper functions for all the natives. Note that in some X11 ports this is already done (module non-static) due to the need to lock the calls going into into XLIB.
 
What are these non-static wrappers going to buy us? Well, if the primary interface of OS.java becomes non-static, then the constructor of OS.java can be protected by doing a permission check with AccessController and friends and throwing exception if the caller's stack does not have enough permissions. The usage pattern of OS.java from within SWT would be like this:
 
In the base class of every SWT, package needing native access, i.e. in Widget.java, Resource.java, Printer.java, etc:
 
<snip>
static OS os;
 
static {
  AccessController.doPrivileged(new PrivilegedAction() {
  public Object run() {
   os = new OS();

    return null;
  }  
 });
   

}
</snip>
 
and then all calls to functions in OS in all classes should be prefixed not with "OS." but with "os."
 
Note that protecting only the constructor is a) sufficient and b) give significant performance advantage, as security checks should not be done on every OS API call; not to mention that having to do the doPrivileged() thing all the time is inconvenient, at best.
 
2. public int Control.handle
 
This is also dangerous, as user is free to change the handle of the control to bogus value, at will. This can be remedied by instead providing:
public int Control.handle()
 
3. Resource tracking.
 
This is nasty. Here's a sample of the problem:
 
<snip>
GC gc = ...;
Font font = ...;
gc.setFont(font);
font.dispose();
gc.drawString("test"); // <-- Crash on platforms based on Toolkits like Fox or probably even GTK+, or weird output on Win32
</snip>
 
The problem here is that after the disposal of the Font instance, there is no valid Font object selected in the GC. In SWT ports wrapping toolkits where Font is represented by a C++ object, this means that the C++ GC class will try to access the C++ Font class, which is already free-d, which leads to memory corruption and possible GPF.
 
The best thing for solving this I've come so far is some sort of resource tracking utility class. The "selection" of every SWT object in another SWT object is tracked there. When the selected object is disposed, the "selector" object is signalled so that it has can switch to using another font handle, usually the default system one which cannot be freed anyway.
 
 
That's it. I know I've missed a tons of stuff. For one thing I haven't covered how in this framework a costom "native" control is supposed to receive native callbacks (WM_* on win32).
 
Regards,
Ivan
 
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